Network Working Group R. P. Swale
Request for Comments: 3304 BTexact Technologies
Category: Informational P. A. Mart
Marconi Communications
P. Sijben
Lucent Technologies
S. Brim
M. Shore
Cisco Systems
August 2002
Middlebox Communications (midcom) Protocol Requirements
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies the requirements that the Middlebox
Communication (midcom) protocol must satisfy in order to meet the
needs of applications wishing to influence the middlebox function.
These requirements were developed with a specific focus on network
address translation and firewall middleboxes.
1. Introduction
This document is one of two developed by the Middlebox Communication
(midcom) working group to address the requirements and framework for
a protocol between middleboxes and "midcom agents." This document
presents midcom requirements; [MCFW] presents the context and
framework. [MCFW] also presents terminology and definitions and
should be read in tandem with this one.
These requirements were developed by examining the midcom framework
and extracting requirements, both explicit and implicit, that
appeared there.
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RFC 3304 Midcom Requirements August 20022. Requirements
Each requirement is presented as a statement, followed by brief
explanatory material as appropriate. Terminology is defined in
[MCFW]. There may be overlap between requirements.
2.1. Protocol machinery
2.1.1.
The Midcom protocol must enable a Midcom agent requiring the services
of a middlebox to establish an authorized association between itself
and the middlebox.
This states that the protocol must allow the middlebox to identify an
agent requesting services and make a determination as to whether or
not the agent will be permitted to do so.
2.1.2.
The Midcom protocol must allow a Midcom agent to communicate with
more than one middlebox simultaneously.
In any but the most simple network, an agent is likely to want to
influence the behavior of more than one middlebox. The protocol
design must not preclude the ability to do this.
2.1.3.
The Midcom protocol must allow a middlebox to communicate with more
than one Midcom agent simultaneously.
There may be multiple instances of a single application or multiple
applications desiring service from a single middlebox, and different
agents may represent them. The protocol design must not preclude the
ability to do so.
2.1.4.
Where a multiplicity of Midcom Agents are interacting with a given
middlebox, the Midcom protocol must provide mechanisms ensuring that
the overall behavior is deterministic.
This states that the protocol must include mechanisms for avoiding
race conditions or other situations in which the requests of one
agent may influence the results of the requests of other agents in an
unpredictable manner.
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RFC 3304 Midcom Requirements August 2002
2.1.5.
The Midcom protocol must enable the middlebox and any associated
Midcom agents to establish a known and stable state. This must
include the case of power failure, or other failure, where the
protocol must ensure that any resources used by a failed element can
be released.
This states that the protocol must provide clear identification for
requests and results and that protocol operations must be atomic with
respect to the midcom protocol.
2.1.6.
The middlebox must be able to report its status to a Midcom agent
with which it is associated.
2.1.7.
The protocol must support unsolicited messages from middlebox to
agent, for reporting conditions detected asynchronously at the
middlebox.
It may be the case that exceptional conditions or other events at the
middlebox (resource shortages, intrusion mitigation) will cause the
middlebox to close pinholes or release resources without consulting
the associated Midcom agent. In that event, the protocol must allow
the middlebox to notify the agent.
2.1.8.
The Midcom protocol must provide for the mutual authentication of
Midcom agent and middlebox to one another.
In addition for the more obvious need for the Midcom agent to
authenticate itself to the middlebox, there are some attacks against
the protocol which can be mitigated by having the middlebox
authenticate to the agent. See [MCFW].
2.1.9.
The Midcom protocol must allow either the Midcom agent or the
middlebox to terminate the Midcom session between a Midcom Agent and
a middlebox. This allows either entity to close the session for
maintenance, security, or other reasons.
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RFC 3304 Midcom Requirements August 2002
2.1.10.
A Midcom agent must be able to determine whether or not a request was
successful.
This states that a middlebox must return a success or failure
indication to a request made by an agent.
2.1.11.
The Midcom protocol must contain version interworking capabilities to
enable subsequent extensions to support different types of middlebox
and future requirements of applications not considered at this stage.
We assume that there will be later revisions of this protocol. The
initial version will focus on communication with firewalls and NATs,
and it is possible that the protocol will need to be modified, as
support for other middlebox types is added. These version
interworking capabilities may include (but are not limited to) a
protocol version number.
2.1.12.
It must be possible to deterministically predict the behavior of the
middlebox in the presence of overlapping rules.
The protocol must preclude nondeterministic behavior in the case of
overlapping rulesets, e.g. by ensuring that some known precedence is
imposed.
2.2. Midcom Protocol Semantics
2.2.1.
The syntax and semantics of the Midcom protocol must be extensible to
allow the requirements of future applications to be adopted.
This is related to, but different from, the requirement for
versioning support. As support for additional middlebox types is
added there may be a need to add new message types.
2.2.2.
The Midcom protocol must support the ability of an agent to install a
ruleset that governs multiple types of middlebox actions (e.g.
firewall and NAT).
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RFC 3304 Midcom Requirements August 2002
This states that a the protocol must support rules and actions for a
variety of types of middleboxes. A Midcom agent ought to be able to
have a single Midcom session with a middlebox and use the Midcom
interface on the middlebox to interface with different middlebox
functions on the same middlebox interface.
2.2.3.
The protocol must support the concept of a ruleset group comprising a
multiple of individual rulesets to be treated as an aggregate.
Applications using more than one data stream may find it more
convenient and more efficient to be able to use single messages to
tear down, extend, and manipulate all middlebox rulesets being used
by one instance of the application.
2.2.4.
The protocol must allow the midcom agent to extend the lifetime of an
existing ruleset that otherwise would be deleted by the middlebox.
2.2.5.
If a peer does not understand an option, it must be clear whether the
action required is to proceed without the unknown attribute being
taken into account or the request is to be rejected. Where
attributes may be ignored if not understood, a means may be provided
to inform the client about what has been ignored.
This states that failure modes must be robust, providing sufficient
information for the agent or middlebox, to be able to accommodate the
failure or to retry with a new option that is more likely to succeed.
2.2.6.
To enable management systems to interact with the Midcom environment,
the protocol must include failure reasons that allow the Midcom Agent
behavior to be modified as a result of the information contained in
the reason. Failure reasons need to be chosen such that they do not
make an attack on security easier.
2.2.7.
The Midcom protocol must not preclude multiple authorized agents from
working on the same ruleset.
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RFC 3304 Midcom Requirements August 2002
2.2.8.
The Midcom protocol must be able to carry filtering rules, including
but not limited to the 5-tuple, from the midcom agent to the
middlebox.
By "5-tuple", we refer to the standard <source address, source port,
destination address, destination port, transport protocol> tuple.
Other filtering elements may be carried, as well.
2.2.9.
When the middlebox performs a port mapping function, the protocol
should allow the Midcom agent to request that the external port
number have the same oddity as the internal port.
This requirement is to support RTP and RTCP [RFC1889] "oddity"
requirements.
2.2.10.
When the middlebox performs a port mapping function, the protocol
should allow the Midcom agent to request that a consecutive range of
external port numbers be mapped to consecutive internal ports. This
requirement is to support RTP and RTCP "sequence" requirements.
2.2.11.
It should be possible to define rulesets that contain a more specific
filter spec than an overlapping ruleset. This should allow agents to
request actions for the subset that contradict those of the
overlapping set.
This should allow a Midcom agent to request to a Midcom server
controlling a firewall function that a subset of the traffic that
would be allowed by the overlapping ruleset be specifically
disallowed.
2.3. General Security Requirements
2.3.1.
The Midcom protocol must provide for message authentication,
confidentiality, and integrity.
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RFC 3304 Midcom Requirements August 2002
2.3.2.
The Midcom protocol must allow for optional confidentiality
protection of control messages. If provided, the mechanism should
allow a choice in the algorithm to be used.
2.3.3.
The Midcom protocol must operate across un-trusted domains, between
the Midcom agent and middlebox in a secure fashion.
2.3.4.
The Midcom protocol must define mechanisms to mitigate replay attacks
on the control messages.
3. Intellectual Property
The IETF takes no position regarding the validity or scope of any
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pertain to the implementation or use other technology described in
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proprietary rights by implementers or users of this specification can
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The IETF invites any interested party to bring to its attention any
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rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
4. Security Considerations
The security requirements for a midcom protocol are discussed in
section 2.3.
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RFC 3304 Midcom Requirements August 2002
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Acknowledgement
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